About your statement in being techinacally IFR in a controlled airspace, in all due respect this is completely wrong.
Flying IFR/VFR and being in a controlled airspace are totally different from each other, one is flying rules while the other is the fact of being subject to ATC authorizations. You can perfectly be flying VFR in a controlled airspace, and flying IFR in an uncontrolled airspace.
I am not "completely wrong" as you put it! Because in the UK and Europe to fly VFR in controlled airspace you need PPR "Prior Permission Required." You can do that on the fly and PF3 allows that. But in practice the real ATC require/prefer the permission to by given before take-off. Once you get permission your flight is designated "Special VFR". The rules in NZ may well allow you to do it but it is not common everywhere else. In fact many countries are very restrictive. You can set 18,000ft in PF3 to be you transition altitude if you want. So in PF3 you can ignore the rules if you so wish.
You are mixing everything up there.
Sorry to insist, I don't want to sound rude or pretentious but I also have an EASA ATPL and I'm a bit aware about how these things work.
Special VFR is a special clearance from ATC when you apply visual flight rules in a controlled airspace and the visibility is lower than 5000 meters and/or the clouds ceiling is below 1500ft, yes this is close to IFR. That said, if the weather is okay you don't need to be a special VFR when you're flying in a controlled airspace, you are flying VFR just like in an uncontrolled airspace, the only difference is that you are subject to the clearance of the ATC for take off, landing, altitude changes etc. but you are still flying VFR !
Special VFR has nothing to do about my "problem", it's just that the ceiling is not high enough.
I don't understand what all of this does here, you are mixing up all the information I gave you and finally I still don't have a solution. I just want to fly a VFR flight plan above 8000 ft as it is possible in real life. For example in France you usually can fly VFR up to FL195, of course it will be in a controlled airspace as soon as you're flying higher than FL115, but as long as you got the clearance, then you can fly VFR in this airspace.
About the transition altitude, I don't want to change it, I'd just like to get back the option to use 18 000ft as being the VFR ceiling instead of using the control center altitude, this would be much easier than changing the control center altitude one by one...
It seems this is the only solution for now. Thanks David for the information.